Given:
Teddy has two sisters. One of them is 3 years younger than Teddy, the other one is 7 years younger than her only brother.
To find:
The age of older sister at the time when the younger sister was born.
Solution:
Let the age of Teddy be x.
One sister is 3 years younger than Teddy.
Age of one sister = x-3 years
The other one is 7 years younger than her only brother. It means other sister is 7 years younger than Teddy.
Age of other sister = x-7 years
Clearly (x-3) > (x-7). So, difference between the ages of sisters is
Therefore, the age of older sister is 4 years when the younger sister was born.